Page:Popular Science Monthly Volume 37.djvu/171

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GLASS-MAKING.

Although, glass is supposed to be a fairly definite chemical compound, each manufacturer has his own notions on the subject, and occasionally he changes his mind, or perhaps his supplies come from a different locality. The result, in either case, would be a slight change in the composition of the batch. A typical mixture would be for every hundred parts of fine white sand about forty parts of alkali (carbonate of soda), ten parts of burned lime, and forty parts of red lead.

It will be noticed that the batch is essentially different from that used in the manufacture of window and Of bottle glass. It differs both in the character and the quality of the materials employed. The ingredients common to the several mixtures must be much purer for use in the production of table and household glassware of the finer grades. Care is taken that the sand shall contain no iron; and, in order to free it from any admixture of loam or other disadvantageous earthy materials, it is subjected to a washing process before it is brought to the mixing-room. By this treatment the more finely divided matter, such as clay and the like, is carried off with the water, while the coarser sand settles to the bottom of the washing-troughs. Further, in the selection of the alkali, the cheaper sulphate of soda is never substituted for the carbonate, as is frequently done in the manufacture of bottles.

In the processes of the atelier the competition is a question of quality rather than of quantity. The element of human labor is so large that it would not be economical to expend it upon an inferior grade of glass. The workers, or rather the men who direct them, go on the principle of those wise domestic economists who reflect that the cost of labor in making up clothing is approximately constant, and who therefore do not feel that they can afford to buy shoddy.

The earthy materials—sand, alkali, and lime—give substance and transparency. Fused together, they form ordinary glass. The additional ingredient, the red lead, has a special function to perform It has for its immediate object an increase in the weight of the glass; and since in general an increase in weight means an increase in refracting power, its ultimate object is an additional brilliancy in the product. Every one has noticed the heaviness of cut glass; or, if he has not, and enters a shop to buy a piece of it, the shop-keeper is very apt to call his attention to the fact—particularly if the price be correspondingly heavy—assuming that weight is an undeniable guarantee of quality and brilliancy. If you object to the price, he puts the piece into your hands and says confidingly, "Just feel the weight of it!" The argument is a pertinent one, but not altogether conclusive, for there are many other elements besides weight upon which the merit of the product depends. It is quite possible to have the glass too heavy for